well... i'm no expert on the subject but i took some soil out of our garden, sifted it, soaked it and dried it a few times and my crypts seem to love it. i probably wouldn't just go from the backyard to the tank though.
If you follow Diana Walstad's el natural method, the backyard soil is just right. But, if you don't follow that method pretty closely, it would be preferable to mineralize the soil first, see the sticky on mineralized toosoil. In either method, you should use a top layer of inert material like pool filter sand, or fine aquarium gravel, or Flourite, to avoid constantly muddy water in the tank.